Do not use av_free() to free avformat_alloc_context() return value in ff_rtp_chain_mux_open()

Description

File: libavformat/rtpenc_chain.c
Function: ff_rtp_chain_mux_open()

Details: In ff_rtp_chain_mux_open(), the return value of avformat_alloc_context() (called at line 44) is freed with av_free() (at line 104). I think the expected paired function is avformat_free_context() (as the one called at line 96). For your convenience, I paste related bugs as follows: